Dump DHT as JSON

[philipbeadle]

Same as for 561 Dump Chain as JSON.

[clarenceb]

As discussed, the existing DHT dump to string contains binary encoded values so the output looks weird.

The proposed JSON output will look something like in this gist: https://gist.github.com/clarenceb/1b301fd802d4a5a33b3cac706153cf1b

Please provide feedback on whether this is ok.

If we can decode the binary values or store them in a more human-readable manner then it would make the DHT JSON dump more useful.

[clarenceb]

I have a working implementation here: clarenceb@8b16cba

But I want to add more test cases and also refactor code to remove duplication in functions dht.String() and dht.JSON() and also dht.DumpIdx() and dht.DumpIdxJSON().

Also, because of weird characters in the entry and link "value" field, I had to create a function to strip out some non-printable chars (see utils.EscapeJSONValue). There might be a nicer way to do this or it might not be necessary if the encoding of this field is changed.

A real DHT dump can be seen in this gist: https://gist.github.com/clarenceb/389e186995b11e29a2206aa67df4458e

[clarenceb]

PR submitted and build passing: #569